1. Field
The subject technology generally relates to wireless communication systems, and more specifically to a method and apparatus for detecting frequency channels in a wireless communication system.
2. Background
Wireless communication systems are widely deployed to provide various communication services such as voice and data communication. These wireless systems may be multiple-access systems capable of communicating with multiple wireless devices (e.g., cellular phones) to provide communication services to multiple users. Wireless communication systems may employ various wireless communication technologies including Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Divisional Multiple Access (FDMA), Orthogonal FDMA (OFDMA), and others. A widely employed example of TDMA is Global System for Mobile communication (GSM). For a given technology, a wireless communication system may transmit and receive signals to and from wireless devices on multiple frequency channels within a frequency band.
A wireless device may be capable of communicating with one or more wireless communication systems. The wireless device may search for available wireless communication systems and frequency channels of the wireless communication systems within its current coverage area. The wireless device may perform this search upon power up, periodically while the wireless device is in an idle mode, and/or when the wireless device loses acquisition of a wireless communication system and/or a frequency channel of the wireless communication system.
To expedite the search for available frequency channels of a wireless communication system, a wireless receiver may first perform a fast search for each frequency channel. In a fast search, the wireless device checks whether receive energy for a frequency channel is above a threshold (e.g., −100 dBm). If the receive energy is above the threshold for the frequency channel, then the wireless device performs a deep search for the frequency channel, which may involve acquiring a pilot signal in the frequency channel, frequency tracking, demodulation, and other operations. Otherwise, the wireless device may discard the frequency channel. The fast search speeds up detection of available frequency channels by screening out frequency channels having receive energy below the threshold. This way, the wireless receiver does not waste time performing a deep search for a frequency channel that is not present. The fast search can greatly reduce the overall search time for frequency channels since a fast search typically takes much less time to perform than a deep search.
However, a fast search may produce a false alarm, in which the fast search falsely detects a frequency channel. This may occur, for example, when two different wireless communication systems have overlapping frequency bands, which can cause one wireless communication system to interfere with the other. For example, a wireless device performing a fast search for a CDMA frequency channel may detect receive energy from a GSM wireless communication system instead, resulting in a false alarm. Due to this false alarm, the wireless device wastes time performing a deep search for the CDMA frequency channel. False alarms may also be caused by other types of interference.
A high rate of false alarms can greatly increase the search time for available frequency channels and diminish the time-saving benefits of the fast search. Also, a false alarm causes a wireless device to consume power performing a deep search for a frequency channel that is not present, which can reduce the battery life of the wireless device.